docs+tools: workflow flowchart harness (P4) + drivers Tier-1/Tier-2 split

- tools/flowgen.py: generates docs/workflows/<name>.md — one outcome-first mermaid
  flowchart per workflow the SDK tests (hpmcu watchdog, modbus read, cru ladder,
  rga offload, relay drive, freshness contract), each stamped with its benchmark
  ns/op or 100% MC/DC result. Deterministic; reads the cargo-bench trend json.
- drivers/README.md: honest Tier-1 (relays + freshness, 100% MC/DC here now) vs
  Tier-2 (modbus/rga — serious testing + fault-injection + benchmarks via sim
  models; driver sources migrate in with the flare-edge unification) per ADR-0002/0005.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

docs/workflows/freshness-contract.md

@@ -0,0 +1,19 @@
+# UI freshness contract (Tier-1, 100% MC/DC)
+
+> **Outcome tested:** The UI never shows a stale number: it holds briefly, then marks unknown.
+
+**Coverage**: freshness.c — 66/66 conditions, 100% MC/DC (CI-enforced)
+
+```mermaid
+flowchart TD
+  A[produce] --> B{result}
+  B -- OK --> V[render value, save last]
+  B -- SAME --> C{showing unknown?}
+  C -- yes --> V
+  C -- no --> N[no change]
+  B -- UNKNOWN --> D{ever had a value?}
+  D -- no --> U[render UNKNOWN mark]
+  D -- yes --> E{age > max_stale?}
+  E -- yes --> U
+  E -- no --> N
+```